The object of devices for artificial weathering material specimens is to estimate the useful life of materials which in application are continually exposed to natural weather conditions and thus degrade under climatic influencing factors such as light and heat of the sun, moisture, and the like. To obtain a good simulation of the natural weather conditions the spectral energy distribution of the light generated in the device needs to correspond as best possible to that of natural sunlight, this being the reason why xenon radiators are employed as the source of radiation in such devices. Accelerated aging testing materials is achieved by irradiating the specimens constantly and with added intensity to speed up aging of the specimens.
The majority of the material specimens tested in artificial weathering devices are made of plastics in which the degradation due to exposure to the weather is mainly caused by the UV component of sunlight. The photochemical primary processes involved, i.e., photon absorption and the generation of energized conditions or free radicals proceed independently of temperature, whereas the subsequent steps in the reaction may be temperature-dependent as a function of the polymers or additives involved and thus the aging of the materials as observed is likewise temperature-dependent.
The aforementioned devices for artificial weathering of specimens comprise as a rule, in addition to the source of radiation, further means with which other artificial weather conditions such as, for example, high humidity, rain or noxious emissions can be generated. In addition to these artificial weathering devices light testing devices also find application which simply contain a source of radiation. Such light testing devices can be used, for example, to determine the sun protection factor or light protection factor of chemical or physical light protection factors such as UV light protection factors. For determining the sun protection factor the spectral energy distribution of the sun is defined from 290 nm to 400 nm. Defined as the standard sun is, e.g., spectral distribution as specified in DIN 67501, whereby the spectral radiation intensity extends down to 10−5/10−6/W/m2. In sun simulators as used in weathering tests such requirements do not exist. The spectral energy distribution of the sun as specified in CIE 85 (Table 4) begins not before 305 nm and is assumed to be 0 at 300 nm, whereby the spectral radiation intensity is of the order of 0.1 W/m2 and higher.
In weathering devices known hitherto usually one or more UV radiation sources such as xenon radiators are made use of. These are appreciated to provide a good simulation of the solar spectrum. But, the emitted radiation comprises relatively high spectral properties in the infrared and UV spectral range which need to be suitable filtered. As regards the UV component, the xenon radiator can be filtered with a WG320 filter of corresponding thickness in thus satisfying the aforementioned requirements on a standard sun for determining the sun protection factor.
Conventional weathering apparatuses have, however, the following drawbacks. Usual xenon radiators having a doped quartz crystal envelope emit radiation with wavelengths up to 250 nm. A commercially available WG320 filter (made by Schott) cannot be bent, this being the reason why strips 10 mm wide and 300 mm long need to be placed together. The radiation can pass at the locations where the filters come together or where metal blanks contact each other. In addition to this, changes in temperature can displace the filter edge. The edge location of the WG320 filter depends on the ambient temperature of the filter (temperature drift 0.06 nm/K). The tolerances on the testing requirements for determining the light protection factor by the COLIPA method are so tight that a change in ambient temperature of 40° K can result in the test being out of tolerance. Since the filters are furthermore arranged in the vicinity of the xenon radiator the ambient temperature for the filters is between 70° C. and 110° C. depending on the output of the lamp. This means that the WG320 filters need to be between 0.7 to 1 mm thick, resulting in the assembly as a whole being highly unstable with strips 10 mm wide and approximately 300 mm long.